discourse/plugins/chat/assets/stylesheets/mobile/chat-composer-upload.scss

.chat-composer-upload {
  &__remove-btn {
    visibility: visible;
    background: rgba(var(--always-black-rgb), 0.9);
    border-radius: 100%;

    // overwrite ios style
    font-size: var(--font-down-2) !important;
    padding: 5px !important;
  }
}